When designing a switch-mode power supply, EMI and electro magnetic compatibility (EMC) are the factors that should be considered.
For a constant frequency control method, frequency jitter is usually applied to lower the EMI energy at a frequency with narrow frequency range. By dithering the switching frequency periodically or non-periodically, the EMI energy may be dispersed in a relatively wide frequency range, so as to meet the EMI standard.
However, for other control methods, the switching frequency is not constant because of the interference from other signals. For an instance, for a constant on-time control method, load change and the oscillation of alternative current (AC) line voltage may both affect the off-time, and further the switching frequency. In addition, the on-time is also affected by the load change and the oscillation of AC line voltage. Therefore, for a system without a constant frequency control method, due to certain characteristics of itself, for example, the ripple of the rectified AC input voltage, the switching frequency may be jittering. But the magnitude of this jittering is not large enough. Thus new frequency jittering structure is required to lower the energy of EMI.